I have heard or read it dozens of times. There are Catholics that assert that the Immaculate Mary did not die. They claim that when Pope Pius XII dogmatically declared the Assumption of Mary, he left the question open. The cite the following from Munificentissimus Deus:
by the authority of our Lord Jesus Christ, of the Blessed Apostles Peter and Paul, and by our own authority, we pronounce, declare, and define it to be a divinely revealed dogma: that the Immaculate Mother of God, the ever Virgin Mary, having completed the course of her earthly life, was assumed body and soul into heavenly glory.
Here, it is said, that the Holy Father left the question open by declaring only "having completed the course of her earthly life," and not "having died."
This begs the question, what did Pope Pius XII intend when he wrote, "having completed the course of her earthly life"? As in the study of Sacred Scripture, the answer lies in context.
If you read Munificentissimus Deus, it becomes manifest that the Holy Father taught that our Immaculate Lady died an earthly death before being assumed bodily into Heaven. This belief is stated repeatedly in the text of Munificentissimus Deus. Here are some examples from Munificentissimus Deus:
Citing Pope Adrian I, His Holiness Pope Pius XII records: "Venerable to us, O Lord, is the festivity of this day on which the holy Mother of God suffered temporal death, but still could not be kept down by the bonds of death, who has begotten your Son our Lord incarnate from herself."
Citing the Byzantine liturgy: "As he kept you a virgin in childbirth, thus he has kept your body incorrupt in the tomb and has glorified it by his divine act of transferring it from the tomb."
Citing Saint Modestus, the Holy Father writes: "As the most glorious Mother of Christ, our Savior and God and the giver of life and immortality, has been endowed with life by him, she has received an eternal incorruptibility of the body together with him who has raised her up from the tomb and has taken her up to himself in a way known only to him."
The citations employed Pope Pius XII reveal that he believed and intended to show that the Immaculate Virgin Mary did in fact undergo death prior to her glorious Assumption.
It should be stated that Mary did not die because of sin, but rather in her desire to be conformed to Christ in all things - to be the speculum justitiae, mirror of justice. Her death gave her dominion over Purgatory as prophesied in Ecclesiasticus 24 and gave her more meritorious prayers for those in the hour of death.
If you would like a detailed defense of the death of the Immaculate Virgin, see the Glories of Mary by Saint Alphonsus Ligouri, a doctor of the Church.